The cycle of the seasons

Most people can hazily remember learning, in science class or from some nature documentary or other, that the earth travels around the sun in a nearly-circular path once per year. Without really thinking about it, it’s easy to suppose that it’s warmer in summer because in the summertime we are a bit closer to the sun than we are in winter.

If you do stop to think about this you realize it can’t be right, because the southern hemisphere has opposite seasons to the northern one; when it’s summer in Australia it’s winter in Russia, and vice versa. Whatever causes the seasons, it has nothing to do with the distance between the earth and the sun.

However it does have something to do with north and south. North and south are the directions of earth’s rotation axis; our planet’s daily rotation about this axis causes the cycle of day and night. But what do north and south have to do with the yearly cycle of the seasons? Why should the northern half of the earth have summer while the southern half has winter, and vice versa?

Fig. 1 Earth’s rotation about its north-south axis causes the cycle of day and night. The yellow circle represents the sun and the blue circle represents the earth, the red line is the equator, and the dotted blue line represents earth’s yearly path around the sun. This diagram is not to scale and there are some other interesting things wrong with it as well, keep reading!

It’s useful to pause a moment and reflect on what the cycle of the seasons actually is:

– One-year cycle
– Summer = warmer, winter = colder
– When it’s summer in the northern hemisphere it’s winter in the southern hemisphere, and vice versa.
– Summer and winter are more pronounced near the poles; near the equator it’s more like there’s a wet season and a dry season, or no seasons at all
– Days are longer in summer and shorter in winter
– This difference in daylight time – longer in summer and shorter in winter – is most extreme near the north and south poles, and just about non-existent near the equator

Looking back at the diagram above, there’s a problem: if you mentally rotate the earth about its north-south axis, you find that every point on earth spends exactly half its time in sunlight, and half its time in darkness; days and nights are of equal length.

But most of the time that’s not how the real world works; most of the time one hemisphere is having longer days and shorter nights, while the other hemisphere is having the opposite. (The exception to this comes twice a year at the equinoxes, when it’s mid-spring on one hemisphere and mid-fall on the other; at these times every part of the earth really does have days and nights of equal length.)

Suppose I redraw the diagram like this:

Fig. 2. The earth is tilted in such a way that the southern hemisphere gets more sunlight overall. The red line is the equator.

In this picture the sunlight is not evenly distributed between the north and south hemispheres; the southern hemisphere gets the lion’s share of sun. And if you imagine the earth in fig. 2 rotating about its north-south axis, you can see that points near the south pole spend nearly all their time in daylight, whereas points near the north pole spend most of their time in darkness. In other words, this picture represents earth when it’s midsummer in the southern hemisphere and midwinter in the northern hemisphere.

Six months later, when earth is at the opposite end of its path around the sun, the situation is reversed: it’s midwinter in the southern hemisphere, and midsummer in the northern hemisphere.

Fig. 3. Earth’s north-south rotation axis is tilted relative to the sun so that the northern hemisphere gets most of the sunlight.

When the earth is between these two extremes, it’s spring in one hemisphere and fall in the other. At these times the north and south hemispheres get roughly equal amounts of sunlight.

Fig. 4. In this diagram it’s slightly past the spring equinox for the northern hemisphere, and slightly past the fall equinox for the southern hemisphere. Both hemispheres get roughly equal amounts of light.

Earth’s north-south rotation axis always points the same way, it doesn’t wobble around.* But as the earth goes around the sun, the angle of earth’s north-south axis relative to the sun does change, and this is what causes the seasons.

* This is a lie: actually the earth’s rotation axis does wobble, but only very slowly. It takes 26,000 years to complete a single wobble, and the amount of wobbling that takes place within a person’s lifetime is too small to notice.